<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    * {
        padding: 0;
        margin: 0;
    }

    .box .left {
        width: 100px;
        height: 80px;
        background-color: aqua;
        float: left;
    }

    .box .right {
        width: 100px;
        height: 80px;
        background-color: blue;
        float: right;
    }

    .box .mian {
        height: 80px;
        background-color: blueviolet;     
        margin-left: 100px;
        margin-right: 100px;
    }
</style>

<body>
    <div class="box">
        <div class="left">1</div>
        <div class="right">2</div>
        <div class="mian">3</div>
    </div>
</body>

</html>